Methods for volumetric tissue ablation

ABSTRACT

A volumetric tissue ablation apparatus includes a probe having a plurality of wires journaled through a catheter with a proximal end connected to the active terminal of a generator and a distal end projecting from a distal end of the catheter. The probe wire distal ends are arranged in an array with the distal ends located generally radially and uniformly spaced-apart from the catheter distal end. A conductor connected to the return terminal of the generator is located relative to the probe wire array to form a closed electrical circuit through tissue to be ablated. Preferably, the probe wire array includes 10 wires, each formed in an arch from the catheter distal end. The conductor can be either a conventional ground plate upon which the tissue is supported, or a conductor wire extending through the probe and electrically insulated from the probe wires.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit ofpriority from application Ser. No. 08/559,072, filed on Nov. 16, 1995,which is a continuation of application Ser. No. 08/410,344, filed onMar. 24, 1995, now U.S. Pat. No. 5,868,740, issued on Feb. 9, 1999. Thepresent application is also related to application Ser. No. 08/766,154,now U.S. Pat. No. 5,855,566 and application Ser. No. 08/764,058, nowU.S. Pat. No. 5,827,276, and to application Ser. Nos. 09/501,338 and09/500,732 both of which were filed on the same day as the presentapplication. The full disclosures of each of these prior and co-pendingapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to radio frequency electrodesfor tissue ablation, and more particularly to an improved RF electrodehaving a spreading array of wires to ablate large volumes of tissue.

BACKGROUND OF THE INVENTION

The liver is a common repository for metastasis from many cancers,including those of the stomach, bowel, pancreas, kidney, and lung. Incolorectal cancer the liver is the initial site of spread in more thanone-third of patients, and is involved in more than two-thirds at thetime of death. While patients with untreated colorectal metastasis tothe liver have no five year survival, patients undergoing surgicalresection have approximately a 25-30% five year survival. Unfortunately,only a limited number of patients are candidates for surgical resection.

Cryosurgery is also used for the treatment of hepatic metastasis.Cryosurgery, which relies on a freeze-thaw process to nonselectivelykill cells, has been found equally effective as surgical resection butis more tissue sparing. While an improvement over open surgical tissueresection, cryosurgery still suffers from disadvantages. It is an opensurgical procedure, requires placement of up to five relatively largeprobes, and can only be applied to a limited number of lesions. Whilepercutaneous probes are being developed, they are currently capable onlyof treatment of smaller lesions. Typical lesions common to colorectalmetastasis, however, are relatively large. Therefore, the outlook forpercutaneous cryotherapy is guarded.

A number of investigators have used radio frequency hyperthermia withplacement of external electrodes, for the treatment of liver cancers.Tumor cells are known to be more sensitive to heat than normal cells,and externally applied regional hyperthermia delivered with radiofrequency tends to ablate the tumor while sparing the normal tissue ofsignificant damage. While this therapy improves the response to systemicchemotherapy, it has uncertain benefit for long-term survival. Onelimitation of hyperthermia is that it is difficult to heat the tumors toa lethally high temperature. Moreover, tumor cells tend to becomethermoresistant if they survive early treatments.

Percutaneous laser hyperthermia has also been used for primary andmetastatic liver cancer. Laser fibers are introduced through needles,under ultrasound guidance. The lesions generated by laser arerepresented by hyperechoic foci on the real time ultrasound images,which can be used to monitor the size of the lesion. Low energy singlefiber systems, which do not require a cooling system along the fiber,can generate areas of necrosis limited to approximately 15 mm diameter.Such small diameters are insufficient for the vast majority of lesionsencountered clinically thus requiring multiple fiber placement andprolonged procedure times.

Radio frequency (RF) hyperthermia, using a standard electrosurgicalgenerator and a fine needle partially sheathed in plastic, has also beenproposed for the treatment of liver and other solid tumors. In onesystem, the apparatus was capable of generating lesions of approximately1×2 cm in a pig liver. In order to produce larger treatment volumes witha single needle, high currents and temperatures have been employed, butproduce charred and carbonized tissue, without enlarging the tissuevolume being treated. To treat a larger lesion, multiple needle passesin different locations would be needed. In preliminary testing, thissystem established a 75% survival at 40 months.

It can therefore be seen that the treatment of primary and metastaticliver tumors and other solid tumors elsewhere in the body, remainsproblematic. Surgery is effective, but only a small percentage ofaffected patients are candidates. Cryotherapy has had improved results;but its applicable patient population is essentially the same as thatfor surgery. The percutaneous methods have the virtue of being lessinvasive, so they can be appropriately used for a larger spectrum ofpatients, but current percutaneous methods all suffer from a limitedability to ablate a large volume of tissue in a single procedure with asingle probe passage.

SUMMARY OF THE INVENTION

It is therefore a general object of the present invention to provide animproved electrosurgical method and probe deployable in a percutaneousprocedure that will produce a large volume of thermally ablated tissuewith a single deployment.

It is a further object that such methods and probes should be useful inopen surgical as well as percutaneous procedures.

Another object is to provide an electrosurgical probe which will provideuniformly treated tissue within a large volumetric lesion.

Still another object of the present invention is to provide apercutaneous electrosurgical probe which requires only a small accesshole but provides for large volumetric tissue ablation.

Still another object is to provide an electrosurgical probe which avoidsthe problems of charring and carbonization common with single needleprobes.

These and other objects will be apparent to those skilled in the art.

The present invention provides both methods and apparatus for the radiofrequency (RF) treatment of a specific region within solid tissue,referred to hereinafter as a “treatment region.” The methods andapparatus rely on introducing at least two electrodes, and usually atleast three electrodes to a target site within the treatment region.After reaching the target site, the plurality of electrodes are deployedwithin the solid tissue, usually in a three-dimensional array andpreferably in a configuration which conforms to or encompasses theentire volume of the treatment region, or as large a portion of thevolume of the treatment region as possible. More preferably, theadjacent electrodes are evenly spaced-apart from each other (i.e., pairsof adjacent electrodes, will be spaced-apart in repeating pattern) sothat application of RF current through the electrodes will result ingenerally uniform heating and necrosis of the entire tissue volume beingtreated. Advantageously, the use of multiple electrodes to treat arelatively large tissue volume allows the RF energy to be applied with alower current density (i.e., from a larger total electrode area) andtherefore at a lower temperature in the tissue immediately surroundingthe electrode. Thus, charring and carbonization of tissue (which hasheretofore been associated with the use of single electrode systems) isreduced. The uniform treatment of a large volume of tissue reduces thenumber of electrode deployments which are necessary for treating atissue region of any given size.

In a first particular aspect, the method of the present inventioncomprises introducing at least two electrodes through solid tissue to atarget site within a treatment region. The at least two electrodes aremaintained in a radially constrained or collapsed configuration as theyare advanced through the tissue to the target site and are then deployedfrom the target site further into the treatment region in a desireddivergent pattern. RF current flow is then established between the atleast two electrodes (i.e., bipolar) or among at least the twoelectrodes and a separate return electrode (i.e., monopolar). Themonopolar return electrode will have a surface area which issufficiently large to dissipate any electrosurgical effect. The at leasttwo electrodes may be deployed by a variety of specific techniques. Forexample, a sheath may be initially placed using an obturator or styletto the target site in a conventional manner. After removing theobturator or stylet, the electrodes can be introduced through the sheathand advanced from the distal end of the sheath into the solid tissue.Optionally, the electrodes may be disposed in or on an elongate member,such as a tube which reciprocatably receives the electrodes. Theelectrodes may then be advanced from the tube, or alternatively the tubemay be withdrawn proximally from over the electrodes prior toadvancement of the electrodes from the sheath into the tissue.

In a second specific aspect, the method of the present inventioncomprises advancing at least three electrodes from a target site withinthe treatment region. The electrodes diverge in a three-dimensionalpattern, preferably with individual electrodes being evenly spaced-apartto provide for uniform volumetric treatment, as discussed above.Treatment is then performed by passing RF current among the at leastthree electrodes or between said three electrodes and a returnelectrode. Preferably, the method will employ more than threeelectrodes, often deploying at least five electrodes, preferablyemploying at least six electrodes, frequently employing at least eightelectrodes, and often employing at least ten electrodes or more. It willbe appreciated that a larger number of individual electrodes can enhancethe uniformity of treatment while limiting the amount of power (currentdensity) emitted from any single electrode, thus reducing thetemperature in the immediate region of the electrode(s). Optionally, theat least three electrodes may be everted, i.e., turned first in aradially outward direction and then in a generally proximal direction,as they are advanced from the target site. The use of such multiple,everted electrodes provides a preferred array for treating relativelylarge tissue volumes. In particular, arrays of everted electrodes willprovide current and heating in generally spherical volumes which willmore closely match the spherical or ellipsoidal geometries of thetypical tumor or other lesion to be treated. In contrast non-evertedelectrode arrays will often effect a conical or irregular treatmentvolume which may have less widespread applicability.

In a first aspect of the apparatus of the present invention, a probesystem comprises an elongate member having a proximal end and a distalend. At least two solid-tissue-penetrating electrode elements arereciprocatably disposed on or in the elongate member so that they may beadvanced into tissue after the elongate member has been introducedthrough solid tissue to a target site in or near the treatment region. Ameans for introducing the elongate member through tissue to the targetsite is also provided. The means may take a variety of forms, includinga sheath and obturator (stylet) assembly which may be used to providethe initial penetration. Alternatively, a self-penetrating element maybe provided directly on the elongate member. Other conventional devicesand techniques of the type used for introducing shafts and otherelongate members to solid tissue may also be employed.

The tissue-penetrating electrode elements may comprise wires which arereceived within an axial lumen of the elongate member. For example, thewires may be bundled together over a proximal portion thereof, butremain separate and shaped over their distal portion so that theydiverge in a selected pattern when advanced into tissue. Usually, thewires will be advanced directly from the elongate member (when theelongate member is left inside the sheath or the sheath is withdrawn),but could alternatively be advanced from the sheath when the elongatemember is withdrawn proximally from over the electrodes prior topenetration of the electrodes into the tissue.

In a second aspect of the apparatus of the present invention, a probesystem comprises an elongate member having a proximal end and a distalend, and at least three solid-tissue penetrating electrode elementsreciprocatably attached to the elongate member. The at least threeelectrodes are configured to diverge in a three-dimensional pattern asthey are advanced in a distal direction from the elongate member.Usually, the elongate member is a tube having an axial lumen whichreciprocatably receives the tissue-penetrating electrode element, andthe electrode elements comprise individual wires which may be bundled asdescribed above. The distal ends of the wires or other electrodeelements are preferably shaped so that they will assume a radiallyconstrained configuration while present in the axial lumen of the tubeand will assume a radially divergent configuration when axially extendedfrom the tube. In a preferred configuration, the distal ends of at leastsome of the wires are shaped so that they assume outwardly evertedconfiguration as they are axially extended from the tube or otherelongate member. The probe system may include one, two, or more groupsof at least three electrodes which are axially spaced-apart from eachother. In particular, such axially spaced-apart groups of electrodes mayextend from the distal end of the elongate member or may be distributedalong the elongate member and individually extendable to assume thedesired three-dimensional configuration. Preferably, each group oftissue-penetrating wires or other electrode elements will include morethan three electrodes, as described generally above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of the tissue ablation apparatus ofthe present invention.

FIG. 2 is an end view of the apparatus of FIG. 1.

FIG. 3 is a sectional view through tissue, showing the prior art affectsof a single needle probe.

FIG. 4 is a sectional view through tissue showing the results of theprobe of the present invention.

FIG. 5 is a side perspective view of a second embodiment of the probe ofthe present invention.

FIG. 6 is a side perspective view of a bipolar embodiment of theinvention.

FIG. 7 is a side perspective view of a second bipolar probe.

FIG. 8 is a side perspective view of a third bipolar probe.

FIGS. 9-14 illustrate use of an exemplary probe system according to thepresent invention in RF treatment of a target region of solid tissue.

GENERAL DESCRIPTION OF THE SYSTEM OF THE PRESENT INVENTION

Systems according to the present invention will be designed to introducea plurality of electrode elements to a treatment region within patientsolid tissue. The treatment region may be located anywhere in the bodywhere hypothermic exposure may be beneficial. Most commonly, thetreatment region will comprise a solid tumor within an organ of thebody, such as the liver, kidney, pancreas, breast, prostate (notaccessed via the urethra), and the like. The volume to be treated willdepend on the size of the tumor or other lesion, typically having atotal volume from 1 cm³ to 150 cm³, usually from 1 cm³ to 50 cm³, andoften from 2 cm² to 35 cm². The peripheral dimensions of the treatmentregion may be regular, e.g., spherical or ellipsoidal, but will moreusually be irregular. The treatment region may be identified usingconventional imaging techniques capable of elucidating a target tissue,e.g., tumor tissue, such as ultrasonic scanning, magnetic resonanceimaging (MRI), computer-assisted tomography (CAT), fluoroscopy, nuclearscanning (using radiolabeled tumor-specific probes), and the like.Preferred is the use of high resolution ultrasound which can be employedto monitor the size and location of the tumor or other lesion beingtreated, either intraoperatively or externally.

Systems according to the present invention will employ a plurality oftissue-penetrating electrodes, typically in the form of sharpened, smalldiameter metal wires which can penetrate into tissue as they areadvanced from a target site within the treatment region, as described inmore detail hereinafter. The electrode elements, however, can also beformed in other manners, such as blades, helices, screws, and the like.The primary requirement of such electrode elements is that they can bedeployed in an array, preferably a three-dimensional array, emanatinggenerally from a target site within the treatment region of the tissue.Generally, the electrode elements will be first introduced to the targetsite in a radially collapsed or other constrained configuration, andthereafter advanced into the tissue from a delivery element in adivergent pattern to achieve the desired three-dimensional array.Preferably, the electrode elements will diverge radially outwardly fromthe delivery element (located at the target site) in a uniform pattern,i.e., with the spacing between adjacent electrodes diverging in asubstantially uniform and/or symmetric pattern. In the exemplaryembodiments, pairs of adjacent electrodes will be spaced-apart from eachother in similar or identical, repeated patterns and will usually besymmetrically positioned about an axis of the delivery element. Theelectrode elements may extend or project along generally straight linesfrom the target site, but will more usually be shaped to curve radiallyoutwardly and optionally to evert proximally so that they face partiallyor fully in the proximal direction when fully deployed. It will beappreciated that a wide variety of particular patterns can be providedto uniformly cover the region to be treated.

A preferred form of the individual electrode element of an electrodearray is a single wire having a shaped distal portion which can beextended from the delivery element at the target site in the tissue todiverge in a desired pattern. Such wires can be formed from conductivemetals having a suitable shape memory, such as stainless steel,nickel-titanium alloys, spring steel alloys, and the like. The wires mayhave circular or non-circular cross-sections, with circular wirestypically having a diameter in the range from about 0.1 mm to 2 mm,preferably from 0.2 mm to 0.5 mm, often from 0.2 mm to 0.3 mm. Thenon-circular wires will usually have equivalent cross-sectional areas.Optionally, the distal ends of the wires may be honed or sharpened tofacilitate their ability to penetrate tissue. The distal ends of suchwires may be hardened using conventional heat treatment or othermetallurgical processes. Such wires may be partially covered withinsulation, although they will be at least partially free frominsulation over their distal portions which will penetrate into thetissue to be treated. In the case of bipolar electrode arrays, it willbe necessary to insulate the positive and negative electrode wires inany regions where they would be in contact with each other during thepower delivery phase. In the case of monopolar arrays, it may bepossible to bundle the wires together with their proximal portionshaving only a single layer of insulation over the entire bundle. Suchbundled wires may be brought out directly to a suitable RF power supply,or may alternatively be connected via other (intermediate) electricalconductors, such as coaxial cable, or the like.

The above-described electrode characteristics apply only to activeelectrodes intended to have the desired surgical effect, i.e., heatingof the surrounding tissue. It will be appreciated that in monopolaroperation, a passive or dispersive “electrode” must also be provided tocomplete the return path for the circuit being created. Such electrodes,which will usually be attached externally to the patient's skin, willhave a much larger area, typically about 130 cm² for an adult, so thatcurrent flux is sufficiently law to avoid significant heating and othersurgical effects. It may also be possible to provide such a dispersivereturn electrode directly on a portion of a sheath or elongate member ofthe system of the present invention, as described in more detail below(generally, when the return electrode is on the sheath, the device willstill be referred to as bipolar).

The RF power supply may be a conventional general purposeelectrosurgical power supply operating at a frequency in the range from400 kHz to 1.2 MHz, with a conventional sinusoidal or non-sinusoidalwave form. Such power supplies are available from many commercialsuppliers, such as Valleylabs, Aspen, Bovie, and Birtcher.

The plurality of electrode elements will usually be contained by orwithin an elongate member which incorporates the delivery element,typically a rigid, metal or plastic cannula. The elongate member servesto constrain the individual electrode elements in a radially collapsedconfiguration to facilitate their introduction to the tissue targetsite. The electrode elements can then be deployed to their desiredconfiguration, usually a three-dimensional configuration, by extendingdistal ends of the electrode elements from the elongate member into thetissue. In the case of the tubular cannula, this can be accomplishedsimply by advancing the distal ends of the electrode elements distallyforward from the tube so that they emerge and deflect (usually as aresult of their own spring memory) in a radially outward pattern.Alternatively, some deflection element or mechanism could be provided onthe elongate member to deflect members with or without shape memory in adesired three-dimensional pattern.

A component or element will be provided for introducing the elongatemember to the target site within the treatment region to be treated. Forexample, a conventional sheath and sharpened obturator (stylet) assemblycan be used to initially access the target site. The assembly can bepositioned under ultrasonic or other conventional imaging, with theobturator/stylet then being removed to leave an access lumen through thesheath. The electrode elements can then be introduced through the sheathlumen, typically while constrained in the elongate member. The electrodeelements are then extended distally beyond the distal end of the sheathinto the treatment region of tissue, and the elongate member cansubsequently be withdrawn or left in place. RF current can then beapplied through the electrodes in either a monopolar or bipolar fashion.With monopolar treatment, a dispersive plate attached externally to thepatient is attached to the other lead from the RF power supply.Alternatively, a return electrode having a relatively large surface areacan be provided on the elongate member, or the sheath. In bipolaroperation, the individual electrode elements can be connectedalternately to the two poles of the RF power supply. Alternatively, oneor more additional electrode elements can be penetrated into the tissueand serve as a common electrode connected at the second pole.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Referring now to the drawings, in which similar or corresponding partsare identified with the same reference numeral, and more particularly toFIG. 1, the volumetric tissue ablation apparatus of the presentinvention is designated generally at 10 and includes a probe 12electrically connected to a generator 14.

In experiments with a prototype of the present invention, the inventorutilized a Bovie®X-10 electrosurgical unit for generator 14, to generateradio frequency current at specific energies, using the probe 12 as theactive electrode and placing the tissue sample on a dispersive or groundplate. Thus, generator 14 includes at least an active terminal 16 and areturn terminal 18, with a dispersive or ground plate 20 electricallyconnected by conductor 22 to terminal 18.

Probe 12 is comprised of a plurality of electrically conductive wires 24which are bundled at a proximal end and connected to terminal 16 toconduct RF current therefrom. Wires 24 are threaded through anelectrically insulated or non-conductive tube or catheter 26.

Wires 24 are preferably formed of spring wire or other material whichwill retain memory. As shown in FIG. 1, a 10-wire array 28 is formedwith each wire 24 arching from catheter 26 in a general “U” shape witheach wire substantially uniformly separated, as shown in FIG. 2. Thus,array 28 is formed of a plurality of wires 24 curving radially outwardlyfrom the axis of distal end 26 a of catheter 26. Wires 24 all extend alength such that a portion of each wire 24 is perpendicular to the axisof tube 26, and preferably continue curving rearwardly back uponthemselves such that wire distal ends 24 a are oriented generallyparallel to the axis of the tube distal end 26 a. As shown in FIG. 1,wire distal ends 24 a generally lay within a plane orthogonal to thetube distal end 26 a, and uniformly spaced-apart from one another.

Because wires 24 are formed of spring steel, they may be drawn withincatheter 26, for percutaneous insertion. Once distal end 26 a ofcatheter 26 is in position, sliding wires 24 through catheter 26 willpermit the memory of the wires to take the radially disposed shape ofthe array 28 shown in FIGS. 1 and 2.

FIG. 3 is a sectional view taken through a liver sample 30, showing theresults of a prior art 18 gauge straight needle 31 with 1.2 cm ofexposed metal when inserted in liver 30 and operated at 20 watts ofpower, with 100% coagulation current, for a period of 5 minutes. As canbe seen in FIG. 3, the lesion 32 produced by the single needle has anarrow elliptical (nearly cylindrical) shape with a diameter ofapproximately 1.2 cm and a length of approximately 2 cm. FIG. 3 alsoshows the effects of very high temperatures near the probe tip with gasformation common with single needle electrosurgical sources, resultingin charred and carbonized tissue 34 immediately around the needle. Thecharring and associated gas formation at the site of the single needleprobes significantly limits the power which may be applied.

FIG. 4 is a sectional view through a liver sample 30′ showing thenecrotic lesion 32′ produced by the 10 wire array 28 of probe 12 of thepresent invention. Probe 12 is located in tissue sample 30′ with tubedistal end 26 a positioned generally centrally at the site at which alesion is desired. Various methods, known in the art, may be utilized toposition probe 12, prior to deployment of wires 24 (shown deployed inhidden lines). Preferably, positioning of tube distal end 26 a isconfirmed by ultrasound or other imaging techniques. Once tube 26 isappropriately positioned, wires 24 are deployed into tissue 30′, thememory of the wire material causing the wire deployment to take apredetermined array shape.

The applicants utilized the same generator 14 at a power of 60 watts,with 100% coagulation current, for a period of 5 minutes. It can be seenthat the necrotic lesion produced by probe 12 is roughly spherical inshape and has a diameter of approximately 3.5 cm. Furthermore, there isno charring evident, indicating no sparking, and a more uniformtemperature distribution within the volume of tissue being treated.During testing, it was found that the temperature of the tissue 2 cmaway from the access of probe 12 at the end of the 5 minutes was 51.4°C. The same 10 wire probe 12 was used repeatedly at the same settingsand produced substantially identical lesions. It was also found that thearea of lethal heating may extend at least another centimeter beyond thevisible lesion shown in FIG. 4, after thermistor measurements were takenduring repeated experiments with probe 12.

While FIGS. 1 and 2 show a general “fountain” shaped array 28 with 10wires 24, various other array designs are equally suitable, utilizinguniform spacing of the wire distal ends 24 a from catheter distal end 26a to produce a symmetrical lesion, or with non-uniform spacing toproduce an asymmetric lesion. For example, as shown in FIG. 5, multiplearrays 28′ may be formed spaced longitudinally from one another. Thisembodiment of the monopolar tissue ablation apparatus is designatedgenerally at 110 and includes a probe 112 electrically connected togenerator 14. Probe 112 includes a first wire bundle 124 journaledthrough a tube 126 with wire distal ends 124 a deployable to form afirst array 28′a extending from tube distal end 126 a. A second wirebundle 125 surrounds tube 126 within an outer tube 127, with wire distalends 125 a deployable to form a second array 28′b projecting from outertube distal end 127 a. The proximal ends 124 b and 125 b of wire bundles124 and 125 are electrically connected in common to active terminal 16.

In operation, outer tube 127 is positioned with distal end 127 a locatedat the predetermined site for the lesion. The second array 28′b is thenformed by deploying wire ends 125 a of second wire bundle 125. Innertube 126 is then moved axially such that tube distal end 126 a is spacedlongitudinally from tube distal end 127 a. First wire bundle 124 is thendeployed such that wire ends 124 a form array 28′a longitudinally spacedfrom array 28′b.

Referring now to FIG. 6, a bipolar embodiment of the tissue ablationapparatus is designated generally at 210 and includes a probe 212electrically connected to a generator 14. Wires 224 are electricallyconnected to terminal 16 on generator 14 and terminate distally in anarray 228 in the same fashion as the array 28 of the first embodiment.However, apparatus 210 includes an integral return path consisting of areturn wire 238 coated with an electrically nonconductive material 236,which extends through catheter 226 within the bundle of wires 224, andhas a distal end 238 a projecting generally centrally within array 228.The proximal end 238 b of wire 238 is connected to return terminal 18,to provide an electrical circuit when probe 212 is deployed withintissue. Thus, a dispersive plate is unnecessary.

Referring now to FIG. 7, a second bipolar embodiment of the tissueablation apparatus is designated generally at 310 and includes a probe312 with wires 324 connected to active terminal 16 of generator 14.Wires 324 project from distal end 326 a of tube 326 to form an array328.

Bipolar apparatus 310 differs from bipolar apparatus 210 of FIG. 6, intwo ways. First, a collar 340 is attached to the exterior of tube distalend 326 a and is electrically connected to return terminal 18 by aconductor 342, to form an electrical return for current supplied bywires 324. Conductor 342 may be affixed to the outside of tube 326, orthreaded through tube 326 while electrically insulted from wires 324.

Second, wires 324 have portions 344 which are coated with anelectrically insulative material. Portions 344 are spaced-apart along aplurality of wires 324 in order to restrict current flow from selectedportions of wires 324 in order to create a more uniform distribution ofheat from the remaining exposed portions of wires 324.

A third bipolar embodiment of the tissue ablation apparatus isdesignated generally at 410 in FIG. 8. Bipolar apparatus 410 includes aprobe 412 with one set of wires 424 connected to one terminal 16′ of acurrent generator 14′, and a second set of wires 425 connected to theopposite terminal 18′. The individual wires of wire bundles 424 and 425have an electrically insulative coating through tube 426, to preventelectrical contact with one another. Wires 424 and 425 preferablyalternate throughout array 428, such that current flows between wires424 and wires 425.

DESCRIPTION OF THE METHOD OF THE PRESENT INVENTION

Referring now to FIGS. 9-14, a treatment region TR within tissue T islocated beneath the skin S of a patient. The treatment region may be asolid tumor or other lesion where it is desired to treat the region byRF hyperthermia. The treatment region TR prior to treatment is shown inFIG. 9.

In order to introduce an electrode array according to the method of thepresent invention, a conventional sheath and obturator/stylet assembly500 is introduced percutaneously (through the skin) so that a distal endof the sheath lies at or within a target site TS, as shown in FIG. 10.Obturator/stylet 504 is then withdrawn from sheath 502, leaving anaccess lumen to the target site, as shown in FIG. 11. A delivery probe510 incorporating the features of the present invention is thenintroduced through the access lumen of the sheath 502 so that a distalend 512 of an outer cannula 515 of the probe lies near the distal end514 of the sheath 502, as shown in FIG. 12. Individual electrodes 520are then extended distally from the distal end 512 of the probe 510 byadvancing cable 516 in the direction of arrow 519, as shown in FIG. 13.The electrodes 520 are advanced so that they first diverge radiallyoutwardly from each other (FIG. 13), eventually everting backward in theproximal direction, as shown in FIG. 14. If desired, the cannula 515 ofprobe 510 is then withdrawn proximally over electrode cable 516, and theelectrode cable is then attached to an RF power supply 518 in amonopolar manner, also as shown in FIG. 14. Radio frequency current maythen be applied from the power supply 518 at a level and for a durationsufficient to raise the temperature of the treatment region TR by adesired amount, typically to a temperature of at least 42° C., usuallyto at least 50° C., for 10 minutes or longer. Higher temperatures willgenerally require much shorter treatment times.

While the method and system just described employs a separate sheath andobturator/stylet assembly 500 for introducing the treatment electrodes,it will be appreciated that the use of such a separate introducer is notnecessary. Alternatively, the electrodes could be introduced through theelongate member, where the elongate member is provided with aself-penetrating element, such as a sharp tip or an electrosurgical tip,to enhance tissue penetration. As a further alternative, a bundle ofelectrodes could be introduced in any constrained fashion (e.g., aremovable ring, soluble sheath, etc.) with the constraint selectivelyreleased after they have reached the target site within the treatmentregion. The present invention thus will encompass use of a variety ofspecific systems for introducing a plurality of electrodes to the targetsite in solid tissue, and thereafter releasing and diverging theindividual electrode elements into a treatment region surrounding thetarget site in a desired three-dimensional array or other configurationor geometry.

Whereas the invention has been shown and described in connection withthe preferred embodiments thereof, many modifications, substitutions andadditions may be made which are within the intended broad scope of theappended claims. It can therefore be seen that the volumetric tissueablation apparatus of the present invention provides an effective anddesirable electrosurgical ablation system which is suitable forpercutaneous and open surgical introduction, produces uniform lesions,and produces lesions large enough to treat a large spectrum of patients.

What is claimed is:
 1. An improved method for the hyperthermic treatmentof a tumor in solid tissue, said method being of the type wherein anelectrode is positioned at the tumor and radio frequency currentdelivered to the tissue to heat and necrose the tumor, wherein theimprovement comprises positioning an array of electrodes at the tumor,wherein the array is configured to produce a lesion having a volume in arange from 1 cm³ to 150 cm³.
 2. An improved method as in claim 1,wherein the volume is in a range from 1 cm³ to 50 cm³.
 3. An improvedmethod as in claim 1, wherein the volume is in the range from 2 cm³ to35 cm³.
 4. An improved method as in claim 1, wherein the tumor is aliver tumor.
 5. An improved method as in any of claims 1 to 4, whereinthe lesion has a generally spherical or ellipsoidal geometry.